CHANGES IN OPTICAL-PROPERTIES OF COALS DURING AIR OXIDATION AT MODERATE TEMPERATURE

Montana Rosebud coal and North Dakota lignite have been oxidized in a fluidized bed reactor at 200 degrees C, with air at 0.2, 0.6 and 1 MPa for reaction times up to 4 h. Oxidized samples were analysed by reflected light microscopy to investigate the variation of reflectance, formation of rims and changes in maceral morphology. The most remarkable changes concerned the vitrinite. Exinite exhibited an increase of reflectance, but this was not measurable owing to the small size of the fragments. The results indicate that the increase of reflectance at 200 degrees C is only partially dependent on chemical changes caused by oxidation. Data show the existence of a threshold oxidation degree, beyond which the reflectance is not affected by further chemical changes occurring in the coal structure. Furthermore, the changes in elemental composition associated with the degree of threshold oxidation are relatively small. During oxidation both Montana Rosebud coal and North Dakota lignite exhibit the formation of rims, but with large differences as regards the size of the phenomenon. The decreased percentage of vitrinite fragments with rims at higher air pressures, and the relationships between the width of rims and reaction time at the three pressures used, support the view that formation of rims occurs when the reaction rate is controlled by the diffusion of oxygen. During reaction, the telinitic structure of vitrinite tends to disappear to assume a collinitic appearance.